An optical waveguide switch is suitable for application in a transmission line changeover device, an external modulator or the like in optical switching equipment, and optical communication networks. The optical waveguide switch (hereinafter simply referred to as "optical switch") employs an optical directional coupler comprising an optical coupling part. The optical coupling part is composed of two optical waveguides disposed adjacent to each other and formed on a crystal substrate displaying an electro-optic effect and light intake/outlet the optical coupling part serving as curved optical waveguides.
Among optical waveguide switches, a polarization-independent optical waveguide switch adaptable for single-mode optical fiber links is particulary desired.
Such an optical waveguide switch is reported in Electronics Letters, Vol. 23, No. 21, pp. 1167-1169 (1987) by M. Kondo et al. In the optical switch, the effective refractive indices of an optical coupling part for two mutually perpendicular polarization components of a light wave propagating through the waveguides are controlled so as to be approximately equal. With such an arrangement, it become possible to carry out a switching operation for a light wave with an arbitrary polarization.
FIG. 1 illustrates structural features of the Kondo et al optical waveguide switch with minor modifications. The optical switch comprises two optical waveguides 2 which form an optical coupling part 2a and light intake/outlet parts 2b, and which are formed by thermally diffusing Ti in to a LiNbO.sub.3 substrate 1. Light path changeover or modulation of a light wave is carried out by applying a voltage to control electrodes 4 provided in the optical coupling part 2a. Each of the optical waveguides 2 comprises a coupling waveguide 21 which provides the coupling part 2a together with another coupling waveguide 21 of the other optical waveguide 2, and curved optical waveguides 22 serving as the intake/outlet parts 2b. The effective refractive indices of both the coupling waveguides 21 are made equal for both ordinary and extraordinary rays (or mutually orthogonal propagation components of light). Thus, complete coupling lengths of the coupling waveguides 21 which are defined as a length enough to completely shift light energy in one coupling waveguide to another through optical coupling are equal for both ordinary and extraordinary rays, resulting in a polarization-independent optical waveguide switch.
The effective refractive index of an optical waveguide is known to be determined by the dimensions of the width and the depth of an optical waveguide and the refractive-index difference between the optical waveguide and a substrate. In addition, it is known that the value of the effective refractive index is greater for greater values of these factors. On the other hand, the difference between the refractive index of the optical waveguide and the refractive index of the LiNbO.sub.3 substrate generally increases with the increase in the concentration of Ti for each of two polarization components (ordinary ray and extraordinary ray) that are mutually orthogonal while the increasing tendency of the refractive-index difference is different from one polarization component to the other (See for example, J. Appl. Phys. 49(9), September 1978, pp. 4677-4682.) Thus, the same refractive-index differences for mutually orthogonal polarization components are achieved at a specific concentration of Ti (referred to as specific Ti concentration hereinafter) in the waveguide. The specific Ti concentration is defined as a Ti concentration in the coupling waveguides for which the differences between the refractive index of the waveguide and that of the substrate are equal for both ordinary and extraordinary rays. The specific Ti concentration depends on the width of the coupling waveguides and the gap G therebetween.
The specific Ti concentration exists in an area where the Ti concentration is relatively low compared with a Ti concentration attained in polarization-dependent optical switches. For instance, according to the referenced work by M. Kondo et al., it is the quantity of thermally diffused Ti of a Ti film with a thickness of 470 .ANG. under the conditions of 1050.degree. C. and 8 hours. In contrast, for a polarization-dependent optical witch, a Ti film with thickness in the range of 700 to 900 .ANG. is diffused under the same temperature and duration of diffusion so that the diffused Ti quantity in the reference is less by 20-45% than the corresponding value for an ordinary or polarization-dependent optical switch. Thus, in the prior polarization-independent optical switch disclosed in the reference, a light wave is confined less strongly to the waveguides than in the prior art polarization-dependent optical switch because the difference between the refractive indices of the Ti-diffused region and the LiNbO.sub.3 substrate is smaller in the former than in the latter.
As a general characteristic of the light wave, radiation loss for curved waveguides is less when its confinement to the curved waveguides is stronger. In other words, by increasing the effective refractive index of the optical waveguides 2, the curvature of the curved portions, namely, the curved light intake/outlet parts 2b of the optical waveguides 2, can be increased. As a result it becomes possible to decrease the device lengths for the optical witch and modulator. For the prior art polarization-independent optical switch shown in FIG. 1, a specific Ti concentration with a small amount of diffused Ti has to be used. Hence, the curvature of the curved light intake part 2b of the optical waveguides cannot be made large in view of the radiation loss. Because of this limitation on the diffused amount of Ti, it is not possible to reduce the device length beyond a certain value. More definitely, it is difficult to realize small device elements while maintaining a loss of less than 1 dB/cm for a light intake part with a radius of curvature of, for example, 40 mm.